The present invention relates to a recording disk data playback apparatus having a capability for playback of both analog and digital recorded data from recording disks, and in particular to a recording disk data playback apparatus which is capable of playback of type of recording disk having a video signal usually with an audio signal recorded thereon by frequency modulation, together with audio data which has been digitized and converted to a pulse train which is superimposed on the frequency modulation signals, with the apparatus being capable of producing output audio signals which are significantly more free from time axis deviations, i.e. jitter, than has been possible in the prior art.
Various types of recording disk data playback apparatus, sometimes referred to as disk players, have been developed. Until recently, such players were divided into two basic types. One type is utilized only for playback of video disks on which a video signal usually with an audio signal is recorded in the form of analog signals. Such disks are sometimes referred to as LDs, and this abbreviation will be used for these in the following. With such disks, the video and analog signals are utilized to frequency-modulate a high-frequency carrier signal, and the resultant modulated signal is recorded on the disk. The other type of disk player is utilized for playback of digital audio disks, sometimes referred to as compact disks or CDs, and the latter abbreviation will be used hereinafter for such disks. CDs have audio data recorded thereon in the form of a digital signal of PCM (pulse-code modulation) type, i.e. a carrier signal is modulated by an encoded digital signal representing the audio data, and the modulated signal is recorded on the disk. However in recent years, a new type of recording disk (designated in the following as LDD) has been developed, as described in Japanese patent 58-45780) whereby an audio signal which has been digitized, e.g. by a method such as PCM, is converted into a pulse train of form suitable for disk recording, e.g. by applying the EFM (eight-to-fourteen) technique, and this pulse train signal is then superimposed upon a signal which has been produced by FM modulation of a high-frequency carrier by a video signal usually with an audio signal. The signal which results from this superimposition of the pulse train signal upon the modulated carrier is recorded on the disk. With the latter method, the audio signal is generally separated into two channels, e.g. corresponding to the stereophonic right and left channels, with 2.3 MHz and 2.8 MHz audio carriers being respectively frequency modulated by the two audio channel signals. The frequency spectrum of the recorded signal is such that the the sync tip portions of the video signal correspond to a frequency of 7.6 MHz, the pedestal level to 8.1 MHz, and the white peak level to 9.3 MHz. If the EFM technique is used to record the audio digital signal, then the frequency spectrum of the pulse train will extend from 3T to 11T, where T is the bit period of the PCM signal, 3T corresponds to a pulse frequency of approximately 720 KHz, and 11T is the maximum pulse width and corresponds to a frequency of approximately 200 KHz. This pulse train signal is superimposed on the main video carrier at a level which is approximately 1/10 off the carrier level, or less. Amplification and slicing close to the zero-crossing points are then performed to produce a pulse-width modulated signal, which is used as the recording signal.
With video and audio signals recorded on a disk by the method described above, the frequency spectrum of the RF signal which is produced from the disk will be as shown in FIG. 1. Here, A denotes the digitized audio signal component, B denotes the audio FM signal component, C denotes the color information component of the video FM signal component, and D denotes the brightness component of the video FM signal component.
A very wide dynamic range, e.g. 90 dB or higher is provided by a digitized audio signal with such a system. Thus, a substantial improvement in acoustic fidelity can be attained, by comparison with recording and playback of audio signals using frequency modulation.
A digital demodulator system for demodulation of a digital signal containing audio data, such as that produced by playback of a digital audio disk, generally incorporates a memory, into which digital data is temporarily stored and then read out, to eliminate the effects of time-axis deviations, e.g. jitter. Prior to being subjected to digital/analog conversion, the digital signal containing the audio data is temporarily written into this memory, with the write-in operation being performed in synchronism with a write-in clock signal of specific frequency. Shortly thereafter, the stored digital data is read out of the memory, in synchronism with a read-out clock signal. This procedure serves to reduce time-axis deviations in the output audio signal after demodulation.
In a recording disk data playback apparatus according to the present invention, a tangential servo loop is also incorporated, which functions during playing of CDs, LDDs and LDs such as to eliminate, as far as possible, time axis deviations (i.e. jitter components) in the playback signal which is reproduced from the disk. This tangential servo loop serves to control movement of a data-sensing light spot along a direction tangential to a disk track. However since this servo loop includes mechanical components such as an actuator, it cannot respond with a sufficient degree of rapidity and accuracy to completely eliminate all jitter components. In addition, due to the fact that due to mechanical tolerances there will be some degree of eccentricity in the rotation of a disk on the apparatus, i.e. disk wobble, a periodic frequency component in the playback will be produced as a result, and this will therefore also add a jitter component to the output audio signal from the apparatus.